1. Technical Field
The present invention relates to an audio amplification apparatus provided with a howling canceller to prevent howling.
2. Related Art
An audio amplification apparatus amplifies an audio signal supplied from a microphone and inputs the amplified signal to a speaker. The audio amplification apparatus forms a closed loop from the speaker to the microphone. It is well known that the audio signal output from the speaker is fed back to the microphone, and repeatedly amplified to cause howling.
To prevent such howling, it has long been proposed that an adaptive filter is used to generate a simulation signal for simulating a feedback audio signal and that a howling canceller is used for the audio amplification apparatus to subtract the simulation signal from an input signal supplied from a microphone (see non-patent document 1). The howling canceller has a delay portion for delaying an audio signal to be input to the speaker. The delay portion provides a delay time to the audio signal corresponding to a traveling time of the audio signal fed back from the speaker to the microphone. The adaptive filter generates a simulation signal by performing a convolution of the delayed signal with an adaptive filter coefficient. An adder portion subtracts the simulation signal from the input signal supplied from the microphone to leave a residual signal that is then supplied to an amplifier portion. The residual signal is amplified by the amplifier portion and is input to the speaker that generates a sound. The adaptive filter is supplied with the residual signal as a reference signal. A known adaptive algorithm (e.g., LMS (Least Mean Square) algorithm) is used to calculate and update the adaptive filter coefficient so as to minimize the residual signal. In this manner, the simulation signal is approximated to the feedback audio signal to prevent the howling.
A known karaoke machine is a kind of the audio amplification apparatus and performs a correlation operation between an audio signal from a sound source such as CD and an input signal from the microphone to find a correlation function for a feedback transmission path from the speaker to the microphone. The karaoke machine uses this correlation function to monitor the degree of risk of howling occurrence. When the risk of howling occurrence is higher than or equal to a specified level, the karaoke machine notifies a user of the risk or decreases the gain of a particular frequency that highly possibly causes the howling.
[Non Patent Document 1] Inazumi, Imai, and Konishi, “Prevention of howling in the audio amplification system using the LMS algorithm,” lecture thesis collection pp. 417-418, The Acoustical Society of Japan, March, 1991.
[Patent Document 1] Japanese Patent Application Laid-Open Publication No. 8-33091
As mentioned above, the conventional howling canceller employing the adaptive filter uses the residual signal to calculate an adaptive filter coefficient so as to minimize a difference between the simulation signal and the feedback audio signal. In this manner, the simulation signal approximates to the feedback audio signal. However, a narrator's audio signal is included in the audio signal supplied from the microphone. The residual signal as a reference supplied to the adaptive filter contains not only a difference between the feedback audio signal and the simulation signal, but also the external audio signal. When such residual signal is used as the reference, it has been difficult to improve the calculation accuracy for calculating the simulation signal approximate to the feedback audio signal. There have been cases of insufficiently preventing the howling.
On the other hand, the conventional karaoke machine for monitoring the risk of howling is configured only to notify the risk of howling and is incapable of preventing the howling. Further, when the configuration aims at decreasing the gain for a frequency that highly possibly causes the howling, there may be a possibility of degrading the quality of reproduction sound generated from the speaker.